1. Field of the Invention
The present invention relates to an image forming apparatus such as a photocopier, printer, facsimile apparatus, composite machine that includes two or more of these, and plotter, and more particularly relates to an electron emission element (including a concept of a field electron emission element) used in a charging device provided in an image forming apparatus or an image display device such as a display or similar.
2. Description of the Background Art
Conventionally, image forming apparatus using the electrophotographic process are known, such as photocopiers, printers, facsimile apparatuses, composite machines that include two or more of these, and plotters. In the electrophotographic process, corona discharge is frequently used in the charge device that uniformly charges a photosensitive member, which is an image carrier, to form a latent image on the image carrier. In the corona discharge method, an electrically conducting case electrode is provided around a platinum or tungsten wire of diameter 50 to 200 μm, or a needle shaped electrode made from stainless steel material or similar. A direct current or alternating current bias voltage is applied between the electrode and the case. Then using the ions formed by ionizing the air molecules around the electrode, the photosensitive member is charged, and uniform charging is possible at a distance.
However, in the corona discharge method, air is ionized, and discharge products such as ozone and oxides of nitrogen (NOx) are generated. It is known that the quantity of ozone and oxides of nitrogen formed after 60 minutes of discharge can each amount to 4 to 10 ppm.
If ozone accumulates at high concentration within an image forming apparatus, the surface of the photosensitive member can become oxidized, the photosensitivity of the photosensitive member can be reduced, the charging capacity can deteriorate, image forming deteriorates, and so on. However, “Development of a corona charger to reduce the deterioration of the photosensitive member due to ozone”, Hisashi Myochin et al., was published in the Journal of the Imaging Society of Japan, No. 31, January 1992. Also, deterioration of parts other than the photosensitive member is accelerated, the life of components is reduced, and other problems are caused.
Also, oxides of nitrogen cause problems as follows. In other words, it is known that oxides of nitrogen are formed by electrical discharge. However, oxides of nitrogen react with moisture in the air to generate sulfuric acid, which then reacts with metals and so on to form metal sulfates. These products have high resistance under a low humidity environment, but under a high humidity environment they react with the moisture in the air, and their resistance becomes low. Therefore, if a thin film of sulfuric acid or sulfates forms on the surface of the photosensitive member, abnormal images are generated, as if the image were flowing. This is caused by absorption of water by sulfuric acid or sulfates and reduction of their resistance, which causes damage to the electrostatic latent image on the surface of the photosensitive member.
Furthermore, oxides of nitrogen remain in the same place without dissociating in the air after discharge. Therefore, compounds formed from oxides of nitrogen adhere to the surface of the photosensitive member, and are generated even when charging is not carried out, in other words, when the process is stopped. Also, as time passes these compounds penetrate into the photosensitive member from the surface, which is one of the causes of degeneration of the photosensitive member. In this case, matter adhering to the surface of the photosensitive member is eliminated by removing the surface of the photosensitive member little by little when cleaning. However, this increases the cost and brings the new problem that it causes deterioration with time.
Also, in the corona discharge method, discharge is carried out at a distance, so the applied voltage is fairly high (4 kV to 10 kV). In addition, the charging voltage varies with charging time. Therefore, to obtain the necessary voltage (400V to 1000V) it is necessary to make the width of the case electrode in the direction of rotation of the photosensitive member large when the speed of the photosensitive member is large. Therefore it is difficult to reduce the size of image forming apparatus having high printing speed.
On the other hand, the close roller charging method is also widely used. In the close roller charging method, a direct or alternating current bias is applied between the photosensitive member and a charging member (charging roller) held close to the photosensitive member. Discharge occurs in the gap between the two, and the photosensitive member is charged. In this charging method, the charging phenomenon in accordance with Paschen's law is used. The required charging voltage is obtained by forming a voltage difference that is larger than the discharge start voltage by the amount of the required charging voltage.
In this case, with the alternating current bias method, the direction of the electric field between the charging member and the photosensitive member alternates with time; discharge and reverse discharge are repeated. With the alternating current bias method, charging is carried out by charging and reverse charging. Although this has the advantage that more uniform charging can be obtained, the hazard (the phenomenon in which the photosensitive member is oxidized by highly oxidizing activated species generated by atmospheric discharge) to the photosensitive member due to discharge is very high.
In this way, charge has been applied to the photosensitive member by some type of charging means that uses Paschen discharge to date. As a result, discharge products formed by the discharge have adhered to the surface of the photosensitive member, and it is not possible to avoid the hazard of the surface of the photosensitive member being oxidized by active species formed by the discharge. Therefore, currently, to reduce the deterioration with time of the image quality and to maintain the image quality, the surface of the photosensitive member is very slightly removed, as described above. On the other hand, this removal consumes the photosensitive member, and from the long term point of view it is preferable to avoid it. However, there is a trade off with preventing the deterioration in image quality due to the photosensitive hazard as described above, so a fundamental solution to the problem is difficult.
Furthermore, there is a contact charging device in which the charging member contacts the photosensitive member when charging the photosensitive member. This is, for example, a roller shaped charging member that charges the photosensitive member while contacting and being rotated by the photosensitive member. With the contact charging method, there is the advantage that the quantity of ozone generated is low compared with the corona discharge method described above, about 0.01 ppm ozone generated after charging for 60 minutes with a direct current applied voltage. Also, the applied voltage is low, so the power supply cost is low, and the electrical insulation design is easy to carry out.
As described in Prior Art 1 (Japanese Patent Application Laid-open No. S57-178257), Prior Art 2 (Japanese Patent Application Laid-open No. S56-104351), Prior Art 3 (Japanese Patent Application Laid-open No. S58-40566), Prior Art 4 (Japanese Patent Application Laid-open No. S58-139156), Prior Art 5 (Japanese Patent Application Laid-open No. S58-150975), and Prior Art 6 (Japanese Patent Application Laid-open No. S63-7380), and so on, in the contact charging method, a narrow gap is formed at the contacting portion or near the contacting portion. Then discharge is formed that can be described by Paschen's law, and the photosensitive member is charged. In these cases, by applying a direct current voltage that is equal to or greater than the voltage to start charging to an electrically conducting member, or, as described in detail in Prior Art 7 (Japanese Patent Application Laid-open No. S63-149669), by applying an oscillating voltage formed by superimposing an alternating current voltage onto a direct current voltage equivalent to the target charging voltage, uniformity of charging can be further promoted.
However, by applying an alternating current voltage, the direction of the electric field between the charging member and the photosensitive member alternate switch time. Discharge and reverse discharge are repeated, and the charging is carried out by the discharge and reverse discharge. This has the advantage that more uniform charging is obtained. On the other hand, as the AC current increases, the generation of ozone and oxides of nitrogen also increase. Depending on the alternating current application conditions, generation of ozone can reach 3 ppm after 60 minutes of charging, a value near to that of the corona discharge method.
Also, on the other hand, as shown in Prior Art 8 (Japanese Patent Application Laid-open No. H8-106200), there is the method of contact injection charging, in which an electrically conducting member to which a voltage is applied contacts the photosensitive member, and charge is injected into trap levels in the surface of the photosensitive member. In this method, a roller shaped electrically conducting member (charging roller) is commonly used as the shape of the electrically conducting member, for ease of control of contact and separation.
However, the charging member that forms the charging roller is made from rubber, so the roller could become deformed if in contact with the photosensitive member for a long time when the image forming apparatus is stopped. Also, rubber can easily absorb moisture, so the fluctuations in electrical resistance as the environment changes are large. Furthermore, to bring out the elasticity of the rubber and to prevent deterioration, several types of plasticizer and activator are necessary. Also, dispersion promotion agent is frequently used to disperse electrically conducting pigment. In other words, the surface of the photosensitive member is made from non-crystalline resin such as polycarbonate or acrylic, so there is the problem that it is very weak with respect to plasticizers and dispersion promotion agent referred to above.
Also, in the contact charging method, if foreign matter comes between the charging member and the photosensitive member, there is the problem that the charging member becomes dirty and charging defects occur. The roller directly contacts the photosensitive member, so if the dirt is held for a long time the photosensitive member becomes dirty. Therefore, there is the problem that image defects such as horizontal shifts or similar are caused.
Therefore, a method that uses an electron emission material has been gaining attention as a charging technology that is different from these technologies. For example, Prior Art 9 (Japanese Patent Art Laid-open No. 2003-145826) describes what is referred to as MIS (Metal Insulator Semiconductor) type and MIM (Metal Insulator Metal) type electron emission elements and an image forming apparatus using them. The MIS type and MIM type electron emission elements have a structure in which an electron emission layer made from insulating material and semiconductor layers, or insulating and metal material layers, is sandwiched between a substrate electrode and a thin film electrode.
Prior Art 10 (Japanese Patent Application Laid-open No. 2001-250467) discloses an electron emission element, a charging device using the electron emission element, and an image forming apparatus. The electron emission element has a structure in which the ends of carbon nanotubes are coated with metal or alloy (a), or at least one of a nitride, carbide, silicide, or boride (b).
Prior Art 11 (Japanese Patent Application Laid-open No. 2002-279885) discloses an electron emission device, a charging device using the electron emission device, and an image forming apparatus. The electron emission device includes a support member made from a quartz, glass, ceramic, metal, silicon or similar substrate; an emitter electrode formed by providing a metal or alloy film on one side of the support member; a plurality of anodized films disposed a predetermined distance above the emitter electrode, formed by anodizing a plurality of aluminum films in an acid such as sulfuric acid, perchloric acid, or similar; a plurality of fine holes formed between each anodized electrode of the plurality of anodized electrodes, having an aperture on the side opposite the emitter electrode; a plurality of carbon nanotubes that emits field electrons, disposed within the fine holes formed between each anodized electrode of the plurality of anodized electrodes, and whose bottom surfaces are in contact with the emitter electrode; and an extractor electrode that covers the apertures of the fine holes, wherein the carbon nanotubes are enclosed by the emitter electrode, the anodized films, and the extractor electrode.
Prior Art 12 (Japanese Patent Application Laid-open No. 2003-140444) discloses an electron emission element, a charging device using the electron emission element, and an image forming apparatus. The electron emission element is formed from a semiconductor layer between an upper electrode and a lower electrode. An organic compound layer is formed by organic compounds adhering to the semiconductor surface of the semiconductor layer.
Other examples of apparatus using an electron emission element include Prior Art 13 (Japanese Patent Application Laid-open No. 2002-311684) and Prior Art 14 (Japanese Patent Application Laid-open No. 2004-327084).
Of the apparatus using electron emission elements referred to above, in recent years there has been much research into carbon nano-materials, including carbon nanotubes which have been widely researched, which suggests a high electron emission capability. For example, in Prior Art 10, it is disclosed that by prescribing the constituent elements at the ends of the carbon nanotubes, the durability of the carbon nanotubes can be improved. In addition, the electron emission element can be used in a non-contacting or contacting type of charging device.
However, carbon nano-materials are organic materials. Therefore, when used for electron emission in the atmosphere, as in electrophotography, the carbon nano-material itself is oxidized by oxygen atoms excited by the emitted electrons, and decomposes by combustion. There is also the problem that carbon nano-material is structurally very weak, so the required lifetime may not be achieved.
Also, when the electron emission elements having the MIS structure or MIM structure disclosed in Prior Art 9 or Prior Art 12 are used, there is the problem that sufficient electron emission cannot be obtained.
Technologies relating to the present invention are also disclosed in, e.g., Prior Art 15 (Japanese Patent No. 3,598,381), Prior Art 16 (Japanese Patent No. 3,119,431), Prior Art 17 (Japanese Patent Application Laid-open No. S62-052866).